Intelligent Agent - Sending email when not enrolled in a group

I want to create an Intelligent Agent that sends an email at set times after checking that a student has not enrolled in a group. I can figure out how to send an email when they have enrolled in a group, but not when they have not. Basically, the idea is to keep reminding the student to do it until either the time runs out or they enroll. Does anyone know how to set this up?

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